Just buy a new muffler and a new header. Start fresh.

Use GREEN PERMANENT Loctite to assemble it. There are 2 types of Green Loctite. You want SLEEVE AND BEARING RETAINER. I think it's stock # 603 (don't quote me on that)

Clean the NEW parts with a good solvent, like brake cleaner and a plastic brush. Clean the engine threads as well. Keep in mind that Loctite sticks to greasy parts like Crisco cooking oil sticks to a door knob (i.e. not at all)

Assemble with the Green Loctite and don't EVER plan on taking it apart again unless you heat it to over 500F with a propane torch. It's PERMANENT if you get ALL the threads clean and assemble it tightly. Give it at least 2 or 3 days to fully cure. It's anaerobic. It cures in the ABSENCE of oxygen.